FIELD OF THE INVENTION
The present invention relates generally to a method of forming a resist pattern, and more specifically, to a method of forming a resist pattern on a semiconductor substrate by means of lithography using far-ultra violet light. The present invention further relates to an organic silane compound for forming an anti-reflection film used in such a method of forming a resist pattern.
Today, in the process of manufacturing 64M Dynamic Random Access Memory devices, development of "quarter micron lithography technology" is in demand. Recently, lithography using far-ultra violet light such as excimer laser light utilizes a resist of the chemical amplification negative type. The resist is formed of a base resin which absorbs little far-ultra violet ray an acid producing agent which is decomposed by photochemical reaction and produces acid and a cross linking agent for cross linking the base resin by acid catalyzed reaction.
FIG. 6 is a representation showing the chemical structural formula of components contained in a resist of the chemical amplification negative type. The resist of the chemical amplification negative type is formed of a base resin of poly-p-hydroxystyrene as shown in FIG. 6 (a), a cross linking agent of melamine type having at least two cross linking points as shown in FIG. 6 (b), and an acid producing agent as shown in FIG. 6 (c). In the figures, n is a natural number representing a degree of polymerization, R represents an alkyl group, M represents a metal element such as arsenic and antimony, and X represents a halogen element.
FIG. 7 is a partially sectional view showing a semiconductor device in the steps in the order of a conventional method of pattern formation.
Referring to FIG. 7 (a), a contact reinforcing layer 9 is formed on a semiconductor substrate 2 for reinforcing the close contact between semiconductor substrate 2 and a resist layer of chemical amplification negative type subsequently to be applied thereon. Contact reinforcing layer 9 is obtained by applying hexamethyl disilazane onto semiconductor substrate 2, and by hard-baking the same.
Resist layer of chemical amplification negative type 3 having a thickness of 1.0 to 1.5 .mu.m, the components of which are shown in FIG. 6, is formed on contact reinforcing layer 9. Resist layer of chemical amplification negative type 3 is formed by spin-coating the surface of semiconductor substrate 2 with resist solution and by soft-baking the same at a temperature around the range from 90.degree. C. to 130.degree. C.
Referring to FIG. 7 (b), excimer laser light 7 is selectively irradiated upon a resist layer of the chemical amplification negative type 3 through a reticle 8. The selective irradiation of excimer laser light 7 allows an agent for generating acid, triphenyl-sulfonium-triflate, to decompose and generate trifluoromethanesulfonic acid and protons 4 in the exposed part 5 of resist layer of chemical amplification negative type 3, as indicated by the reaction formula in FIG. 8.
Referring to FIG. 7 (c), baking after the exposure is conducted for about a few minutes at a temperature around the range from 110.degree. to 140.degree. C. Description will be given on the state of cross linking in the exposed part of the base resin.
FIG. 9 (a) shows how excimer laser light irradiates resist 3. When the excimer laser light irradiates resist 3, referring to FIG. 9 (b), triphenyl-sulfonium-triflate decomposes to generate acid (H.sup.+). Referring to FIG. 9 (c), baking of the resist causes one chain of the base resin to be cross linked in the presence of catalyst acid (H.sup.+), and acid (H.sup.+) is formed as a by-product. Referring to FIG. 9 (d), in the presence of the acid (H.sup.+) as catalyst formed as the by-product, base resins are cross-coupled one after another in a chain reaction. Referring to FIG. 9 (e), the above-stated chain reaction produces a network of crosslinked base resins. The cross linking reaction is represented by the reaction formula shown in FIG. 10. In the reaction formula, HX represents acid (H.sup.+). The crosslinked part becomes insoluble in a developing agent.
Referring back to FIG. 7 (c), the cross-linked part 6 of the resist becomes insoluble in a developing agent. Referring to FIG. 7 (d), when resist film 3 is developed with an alkaline developing agent of a suitable concentration, the non-exposed part (the part not crosslinked) dissolves by the developing agent, and a resist pattern is formed on semiconductor substrate 2. Then, semiconductor substrate 2 is etched, using the resist pattern 100 as a mask.
According to a conventional method of forming a resist pattern as described above, a very small good resist pattern having a rectangular cross section is produced with high sensitivity on a flat silicon substrate.
However, the base resin, poly-p-hydroxystyrene has high transmittance to excimer laser light and is therefore highly susceptible to intra film multiple reflection effect due to the excimer laser light 7 reflected from the underlying semiconductor substrate 2. As shown in FIG. 11, the intra film multiple reflection effect is caused by the coherence between the irradiated light 17 and the light reflected from the underlying semiconductor substrate 2. Referring to FIG. 12, a change in the thickness of the resist due to this intra film multiple reflection effect results in a great change in the size of a resultant resist pattern. Referring to FIG. 11, a step portion 2a existing in semiconductor substrate 2 causes a change in the thickness of resist 3, and the size of the resist pattern is not constant as a result.
A possible approach to prevent this intra film multi reflection effect uses an organic antireflection film. An organic antireflection film is formed by applying resist of novolak-naphthoquinonediazide type onto a semiconductor substrate, prior to application of the chemical amplification negative type resist and then by hard-baking the same. This method however bears the following problem.
Referring to FIG. 13 (a), when resist of novolak-naphthoquinonediazide type 30 is applied as a thin layer on semiconductor substrate 2 having step 2a, the corner part 21 of step 2a is exposed for the lack of chemical affinity of semiconductor substrate 2 and resist of novalak-napthoquinonediziade type 30. In order to apply the resist of novalak-napthoquinonediziade type 30 so as to cover corner part 21, as shown in FIGS. 13 (b) and (c), the thickness of the resist becomes considerably large in the bottom part of step 2a. If, for example, step 2a is as thick as 0.7 .mu.m, the resist of novalak-naphthoquinonediazide type 30 in the bottom part of step 2a attains a thickness of about 1.5 .mu.m.
The antireflection film is completed by hard baking the resist of novalak-naphthoquinonediazide type 30 applied as thick as this. Thereafter, referring to FIG. 13 (d) resist for lithography 101 is applied onto antireflection film (30), and patterning is performed thereon. Then, using the patterned resist for lithography 101 as a mask, semiconductor substrate 2 and the antireflection film are etched at a time. At that time, there is almost no difference in the etching speeds of antireflection film 30 and resist for lithography 101. Referring to FIG. 13 (e), the size of semiconductor substrate 2 cannot be controlled successively in the process of etching the same.